The systemic effects of caffeine have not been well delineated in conjunction with physical and psychological stress in humans. Either caffeine or stressor challenge alone is capable of influencing cardiovascular function, neurohormonal output, and energy mobilization. The two in combination may interact on these functions, perhaps enhancing each other's effects. The aims of this project are to study how caffeine modifies these responses to psychological and physical challenge both in healthy young men and in those at risk for the development of hypertension. The proposed research will therefore examine psychological and physical challenges in combination with moderate levels of caffeine in a pair of counterbalanced, placebo-controlled, double-blind, corssover studies on these two subject groups. Psychological challenge will be induced by having subjects work under a high state of vigilance on a very demanding mental task to attain monetary reward. Physical challenges will consist of pedalling a bicycle ergometer at each level of a graded series of workloads and isometric handgrip. Dependent variables are: Cardiovascular - heart rate, systemic blood pressures, stroke volume, cardiac output, end-systolic and end-diastolic volumes, ejection fractions, peripheral resistance, and systolic time intervals; Neuroendocrine - plasma concentrations of norepinephrine, epinerphrine, and cortisol; and Energy Mobilization - plasma free fatty acid levels and glucose. The longterm objective of this work is to establish a base of knowledge on the role caffeine may play in modifying physiological responses to stress in nromals and in persons at risk of developing cardiovascular diseases such as hypertension. Such information would be useful in determining whether reduction in caffeine use has any potential in disease prevention either for the population as a whole or for certain targeted groups such as borderline hypertensives.